1. Field of the Invention
This invention relates generally to a filter for sifting and channeling material flowing out of a bottom-unloading tower silo, and relates more particularly to a filtering flow box for receiving plastic pellets flowing from a bottom-unloading tower silo, sifting unwanted plastic strands (“angel hair”) out of the supply of pellets, and channeling the sifted pellets into vacuum feed lines for further processing.
2. Description of the Related Art
Plastic pellets comprise the raw material for many plastic articles of manufacture. Because of their small size and granular nature, such pellets are preferred for their ease of handling and processing. In order to convert plastic pellets into a form that is useable for fabrication, the pellets are melted down, and the resulting molten plastic is subsequently used in processes such as injection molding or blow molding.
Typically, plastic pellets are stored in bottom-unloading tower silos adjacent a processing facility. The pellets are conveyed into the processing facility through vacuum feed-lines when needed. One problem associated with plastic pellets is the formation of so-called “angel hair.” Angel hair is thin, elongated plastic strands that result from the unraveling of plastic pellets. During transportation and handling, it is common for a significant quantity of pellets to unravel and form angel hair. Angel hair is generally undesirable because they are not useful for processing, and therefore must be removed from a supply of pellets before use.
Traditionally, the problem of angel hair has been dealt with by mounting a filter box below the outlet of a bottom-unloading tower silo in which plastic pellets are stored. A filter box is generally rectangular in shape and has four side walls, an open top for receiving material, and an open bottom for dispensing material. The top of the box has a flange that is adapted to bolt directly to the edges of an opening on the bottom of the silo. Thus, all of the pellets that fall through the opening (outlet) of the silo must subsequently pass into the attached filter box before being used.
The conventional filter box has a narrow, horizontal slot formed in one of its side walls for accepting a flat, rectangular filter that is formed of slit and expanded metal. Brackets extend from the inner surfaces of the side walls for holding the filter in a perpendicular orientation relative to the flow of material through the box. As plastic pellets fall by gravity from the silo into the box, the pellets, which are slightly smaller than the apertures in the filter, are allowed to pass through the box while most of the angel hair is caught by the filter.
Over time, as angel hair accumulates on the upstream surface of the filter, the apertures of the filter become increasingly clogged and the throughput of pellets steadily decreases. Once the apertures are sufficiently blocked, pellets settle on top of the collected angel hair and cannot pass through. An operator must therefore periodically pull the filter out of the filter box and manually remove the accumulated angel hair from the filter's upstream surface.
A problem that is commonly associated with the procedure of removing and cleaning the filter of a traditional filter box is that a quantity of pellets that accumulate on top of a clogged filter tend to remain on the filter when it is removed from the box. Thus, as an operator pulls the filter out of the box, those remaining pellets roll off the top of the filter and fall to the ground or floor adjacent the operator, creating a mess and a potential slip hazard for the operator and others who may be in the area. Cleaning up the dropped pellets is a tedious and time consuming task.
Once the plastic pellets have been sifted through the filter box, they fall into a flow box that is mounted to the bottom of the filter box. The flow box is similar to the filter box in size and shape, but has a closed bottom. Vacuum feed-lines are connected to outlet ports formed in a side wall of the flow box. The feed lines remove pellets that accumulate in the box and convey them into an adjacent processing facility.